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 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * CS licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package org.orekit.propagation.integration;

import org.hipparchus.CalculusFieldElement;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.time.FieldAbsoluteDate;

/** Provider for additional derivatives.
*
* <p>
* In some cases users may need to integrate some problem-specific equations along with
* classical spacecraft equations of motions. One example is optimal control in low
* thrust where adjoint parameters linked to the minimized Hamiltonian must be integrated.
* Another example is formation flying or rendez-vous which use the Clohessy-Whiltshire
* equations for the relative motion.
* </p>
* <p>
* This interface allows users to add such equations to a {@link
* org.orekit.propagation.numerical.FieldNumericalPropagator numerical propagator} or a {@link
* org.orekit.propagation.semianalytical.dsst.FieldDSSTPropagator DSST propagator}. Users provide the
* equations as an implementation of this interface and register it to the propagator thanks to
* its {@link FieldAbstractIntegratedPropagator#addAdditionalDerivativesProvider(FieldAdditionalDerivativesProvider)}
* method. Several such objects can be registered with each numerical propagator, but it is
* recommended to gather in the same object the sets of parameters which equations can interact
* on each others states.
* </p>
* <p>
* This interface is the numerical (read not already integrated) counterpart of
* the {@link org.orekit.propagation.FieldAdditionalStateProvider} interface.
* It allows to append various additional state parameters to any {@link
* org.orekit.propagation.numerical.FieldNumericalPropagator numerical propagator} or {@link
* org.orekit.propagation.semianalytical.dsst.FieldDSSTPropagator DSST propagator}.
* </p>
* @see org.orekit.propagation.integration.FieldAbstractIntegratedPropagator
* @author Luc Maisonobe
* @since 11.1
* @param <T> type of the field elements
*/
public interface FieldAdditionalDerivativesProvider<T extends CalculusFieldElement<T>> {

    /** Get the name of the additional derivatives (which will become state once integrated).
     * @return name of the additional state (names containing "orekit"
     * with any case are reserved for the library internal use)
     */
    String getName();

    /** Get the dimension of the generated derivative.
     * @return dimension of the generated
     */
    int getDimension();

    /** Initialize the generator at the start of propagation.
     * @param initialState initial state information at the start of propagation
     * @param target       date of propagation
     */
    default void init(final FieldSpacecraftState<T> initialState, final FieldAbsoluteDate<T> target) {
        // nothing by default
    }

    /** Check if this provider should yield so another provider has an opportunity to add missing parts.
     * <p>
     * Decision to yield is often based on an additional state being {@link FieldSpacecraftState#hasAdditionalState(String)
     * already available} in the provided {@code state} (but it could theoretically also depend on
     * an additional state derivative being {@link FieldSpacecraftState#hasAdditionalStateDerivative(String)
     * already available}, or any other criterion). If for example a provider needs the state transition
     * matrix, it could implement this method as:
     * </p>
     * <pre>{@code
     * public boolean yields(final FieldSpacecraftState<T> state) {
     *     return !state.getAdditionalStates().containsKey("STM");
     * }
     * }</pre>
     * <p>
     * The default implementation returns {@code false}, meaning that derivative data can be
     * {@link #combinedDerivatives(FieldSpacecraftState) computed} immediately.
     * </p>
     * @param state state to handle
     * @return true if this provider should yield so another provider has an opportunity to add missing parts
     * as the state is incrementally built up
     */
    default boolean yields(FieldSpacecraftState<T> state) {
        return false;
    }

    /** Compute the derivatives related to the additional state (and optionally main state increments).
     * @param s current state information: date, kinematics, attitude, and
     * additional states this equations depend on (according to the
     * {@link #yields(FieldSpacecraftState) yields} method)
     * @return computed combined derivatives, which may include some incremental
     * coupling effect to add to main state derivatives
     * @since 11.2
     */
    FieldCombinedDerivatives<T> combinedDerivatives(FieldSpacecraftState<T> s);

}
